A power amplifier provided in the output section of a transmitting apparatus in a wireless communication system requires both low distortion and efficiency. Power amplifiers are classified into amplifiers using transistors for the current source or using transistors for switching operation. Amplifiers using transistors for the current source may be class A amplifiers, class AB amplifiers, class B amplifiers and class C amplifiers. Further, amplifiers using transistors for switching operation may be class D amplifiers, class E amplifiers and class F amplifiers.
Conventionally, in high-frequency power amplifiers that amplify modulation waves including envelope fluctuation components, class A or class AB linear amplifiers are used in order to linearly amplify envelope fluctuation components. However, there is a drawback that the power efficiency of linear amplifiers is inferior compared to non-linear amplifiers such as class C amplifiers or class E amplifiers.
As a result, when a conventional linear amplifier is used for a mobile type wireless apparatus such as a mobile telephone or mobile information terminal apparatus using a battery as a power source, there is the drawback that usage time is short. Further, when a conventional linear amplifier is used for a base station apparatus in a mobile communication system provided with a plurality of large power transmission apparatuses, there is a drawback of causing an increase in the size of the base station apparatus and the heating value.
Therefore, as a transmission apparatus with efficient transmission functions, there is a transmission apparatus that attaches an envelope component to a phase modulation wave by dividing a modulation signal into the amplitude component (envelope component) and the phase component, generating a phase modulation wave with constant envelope from the phase component and changing the power supply voltage of a non-linear amplifier according to the envelope component and that causes a vector modulation wave including an envelope fluctuation component.
FIG. 1 illustrates a configuration of a transmitting apparatus as a first conventional example described above. This transmitting apparatus 10 as the first conventional example is formed with transmission data signal input terminal 11, I signal generating section 13, Q signal generating section 14, amplitude signal voltage generating section 15, phase modulation wave generating section 16, power amplifying section 17 and transmission output terminal 18. I signal generating section 13 and Q signal generating section 14 form complex envelope computing section 12.
In wireless communication system, a transmission signal with information to be transmitted is transmitted as a modulation wave by means of amplitude r(t) and phase φ(t) of a carrier wave. In this case, if the angular frequency of carrier wave is ωc, modulation wave S(t) to be transmitted to a channel is expressed by the following equation.S(t)=r(t)·exp[ωc·t+φ(t)]  (Equation 1)
Here, phase modulation wave Sc(t) with a constant envelope is expressed by the following equation.Sc(t)=exp[ωc·t+ω(t)]  (Equation 2)
In the modulation step in signal processing, a transmission data signal is often processed as the I signal I(t) and the Q signal (Q) that are the in-phase component and quadrature component of complex envelope. These are collectively referred to as “IQ signals” and are expressed by the following equations.I(t)=r(t)·cos [φ(t)]  (Equation 3)Q(t)=r(t)·sin [φ(t)]  (Equation 4)
Further, if an amplitude signal r(t) and a phase signal φ(t) are expressed by IQ signals, equations are as follows.r(t)={I(t)2+Q(t)2}1/2  (Equation 5)φ(t)=tan−1{Q(t)/I(t)}  (Equation 6)
In FIG. 1, a transmission data signal inputted from transmission data signal input terminal 11 is converted into an I signal (I)t in I signal generating section 13 and a Q signal Q(t) in Q signal generating section 14.
Amplitude signal voltage generating section 15 and phase modulation wave generating section 16 generate an amplitude signal r(t) and phase modulation wave Sc(t) from the I signal I(t) and the Q signal Q(t), respectively.
Phase modulation wave Sc(t) that is a carrier wave having angular frequency ωc and phase-modulated by the phase signal φ(t), is generated in phase modulation wave generating section 16 and inputted to power amplifying section 17.
On the other hand, the amplitude signal r(t) is used to set a power supply voltage value of power amplifying section 17. The amplitude of the output signal of power amplifying section 17 changes according to the power supply voltage value. That is, the envelope of the output signal of power amplifying section 17 changes according to the amplification signal r(t).
As a result, the signal acquired by multiplying power supply voltage value r(t) of power amplifying section 17 and phase modulation wave Sc(t), which is the output signal of phase modulation wave generating section 16, is amplified by gain G of power amplifying section 17, and outputted as RF vector modulation wave Srf(t). That is, vector modulation wave Srt(t) is expressed by the following equation.Srf(t)=G·r(t)·exp [ωc·t+φ(t)]  (Equation 7)
As described above, the modulation wave to be inputted to the input terminal of power amplifying section 17 is phase modulation wave Sc(t) with a fixed envelope level, and, consequently, an efficient non-linear amplifier can be used for power amplifying section 17 as a high frequency amplifier, so that it is possible to provide a efficient transmitting apparatus (for example, see Patent Document 1).
FIG. 2 shows a configuration example of amplitude signal voltage generating section 15 and phase modulation generating section 16 of transmitting apparatus 20 in the first conventional example. In amplitude signal voltage generating section 15, amplitude signal generating section 21 computes and generates an amplitude signal r(t) by equation 5 and DA converter 22 generates amplitude signal voltage. In phase modulation wave generating section 16, phase signal generating section 23 computes and generates a phase signal φ(t) by equation 6, DA converter 24 generates phase signal voltage and phase modulating section 25 phase-modulates a carrier wave by the phase signal φ(t) to form phase modulation wave Sc(t).
FIG. 3 is a block diagram for further explanation of the first conventional example. In FIG. 3, the configurations of I signal generating section 13 and Q signal generating section 14 forming complex envelope computing section 12 are shown in detail.
Recently, many wireless communication systems employ a digital modulation scheme. In this case, information to be transmitted is a digital value, and, consequently, a transmission data signal can be expressed by a discrete-time pulse sequence. However, this pulse shape signal excessively occupies a wide frequency bandwidth and therefore is generally processed as continuous-time, smoothed waveforms I(t) and Q(t) by band-limiting discrete-time pulse shape waveforms Ip(t) and Qp(t) in filters. In this case, for the filter, a pulse shaping filter (for example, root cosine roll-off filter) that is directed to performing band-limitation while canceling the intersymbol interference between pulses, is used.
Therefore, in transmitting apparatus 30 shown in FIG. 3, when I signal generating section 13 and Q signal generating section 14 receives as input a transmission data signal from transmission data signal input terminal 11, first, pulse generating sections 31 and 32 convert the transmission data signal into an I component pulse signal Ip(t) and a Q component pulse signal Qp(t), respectively, and pulse shaping filters 33 and 34 perform band-limitation and outputs the I signal I(t) and the Q signal Q(t), respectively.
Although digital filters are generally used for pulse shaping filters 33 and 34 in most cases, analogue filters may be used.
Further, when digital filters are used for pulse shaping filters 33 and 34, although FIR filters (finite impulse response filters) are generally used in most cases, IIR filters (infinite impulse response filters) can be used as well.
For example, when an FIR filter is used and the order of transfer function is m+1 (i.e., the number of taps is m+1), if the coefficient of the transfer function (i.e., tap coefficient) is Ck (here, k is a natural number), the relationship between input signal data sequence X(n) and output signal data sequence Y(n) can be expressed by the following equation.Y(n)=C0·X(n)+C1·X(n−1)+ . . . +Cm·X(n−m)  (Equation 8)
By the way, when there is a difference between the time amplitude signal r(t) takes to reach power amplifying section 17 and the time phase signal φ(t) takes to reach power amplifying section 17, distortion occurs in vector modulation wave Srf (t), which is the output signal of power amplifying section 17. This time difference is caused by, e.g., the difference of delay time in circuits that process an amplitude signal r(t) and a phase signal φ(t).
FIG. 4 is a graph showing frequency spectrum of vector frequency Srf(t) that is the output signal of power amplifying section 17. In the paths through which an amplitude signal r(t) and a phase signal φ(t) reach power amplifying section 17, if there is no difference between delay times, the frequency spectrum shown in FIG. 4A can be acquired, and, if there is a difference between the delay times, a frequency spectrum spread (distortion of modulation wave) occurs as shown in FIG. 4B. In a frequency division multiplex communication system where a plurality of channels are arranged in the frequency domain, this frequency spectrum spread interferes with adjacent channels and therefore is not desirable.
FIG. 5 is a block diagram showing the configuration of transmitting apparatus 40 employs a configuration correcting the difference between the time amplitude signal r(t) takes to reach power amplifying section 17 and the time phase signal φ(t) takes to reach power amplifying section 17, as a second conventional example. This transmitting apparatus 40 of the second conventional example provides shift registers 41, 42, 43 and 44 in the subsequent stage of pulse shaping filters 33 and 34.
Operations of transmitting apparatus 40 of the conventional example will be explained. In addition to the same operations as in transmitting apparatus 30 of the first conventional example shown in FIG. 3, by delaying an IQ signal by given time in shift registers 41 to 44, transmitting apparatus 40 of the conventional example delays an amplitude signal r(t) or a phase signal φ(t) by given time. The delay time is set to the given time by, for example, selecting a given output stage, by a switch from a plurality of connected registers and extracting a signal. Thus, by adjusting the difference between delay times in shift registers 41 and 43 provided on the path to generate an amplitude signal r(t) and the delay time in shift registers 42 and 44 provided on the path to generate a phase signal φ(t), and by reducing the difference between the time amplitude signal r(t) takes to reach power amplifying section 17 and the time phase signal takes to reach power amplifying section 17, it is possible to reduce the distortion of vector modulation wave Srf(t) (e.g., see Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open No. Hei 6-54877